1. Field of the Invention
The invention relates to a method for determining switching state of a transistor-based switching device, more particularly to a method for determining effectively the switching state of a transistor-based switching device.
2. Description of the Related Art
FIG. 1 illustrates a simplified transistor-based switching device 1 that includes first and second sets of field-effect transistors 9, 10, a controller circuit 11 (e.g., a PWM or a PFM controller in a switching power supply), first and second logic circuits 12, 13, and first and second driving circuits 14, 15.
The field-effect transistors in each of the first and second sets 9, 10 are connected in parallel.
The first driving circuit 14 has input and output nodes connected electrically and respectively to the first logic circuit 12 and the gates of the field-effect transistors in the first set 9, whereas the second driving circuit 15 has input and output nodes connected electrically and respectively to the second logic circuit and the gates of the field-effect transistors in the second set 10.
The first and second logic circuits 12, 13 are connected electrically to the controller circuit 11. The first logic circuit 12 is further connected electrically across gate and source of the field-effect transistor in the second set 10 closest to the output node of the second driving circuit 15. The second logic circuit 13 is further connected electrically a cross gate and source of the field-effect transistor in the first set 9 closest to the output node of the first driving circuit 14.
In operation, the first driving circuit 14 applies a voltage to the gates of the field-effect transistors in the first set 9 so as to dispose the transistors in the first set 9 in a non-conducting state. The second logic circuit 13 then detects a voltage level across the gates and sources of the field-effect transistors in the first set 9, and compares the detected voltage level with a predetermined threshold voltage level. When the detected voltage level is found to be lower than the predetermined threshold voltage level, the second driving circuit 15 applies a voltage to the gates of the field-effect transistors in the second set 10 so as to dispose the transistors in the second set 10 in a conducting state. After a predetermined period, the second driving circuit 15 applies the voltage to the gates of the field-effect transistors in the second set 10 so as to dispose the field-effect transistors in the second set 10 in a non-conducting state. The first logic circuit 12 then detects a voltage level across the gates and sources of the field-effect transistors in the second set 10, and compares the detected voltage level with the predetermined threshold voltage level. When the detected voltage level is found to be lower than the predetermined threshold voltage level, the first driving circuit 14 applies the voltage to the gates of the field-effect transistors in the first set 9 so as to dispose the field-effect transistors in the first set 9 in a conducting state. After the predetermined period, the whole operation is repeated.
The problem with the simplified transistor-based switching device 1 is that, since the gates of the field-effect transistors are disposed at different distances from the output node of the corresponding driving circuit 14, 15, the field-effect transistors in each of the first and second sets 9, 10 have different actual response times to the bias voltage. Since the voltage level is detected across the gate and source of the transistor in the first (second) set 9 (10) closest to the output node of the second (first) driving circuit 15 (14), the detected voltage level is not an accurate indication of the transistor state of all the field-effect transistors in each of the first (second) set 9 (10).
In order to solve the above problem, with further reference to FIG. 2, a conventional transistor-based switching device 1′ further includes first and second delay circuits 16, 17, and first and second detecting circuits 18, 19.
Each of the first and second delay circuits 16, 17 is connected electrically to a respective one of the first and second logic circuits 12, 13.
The first detecting circuit 18 is connected electrically to the first delay circuit 16 and across the gate and source of the field-effect transistor in the second set 10 closest to the output node of the second driving circuit 15, whereas the second detecting circuit 19 is connected electrically to the second delay circuit 17 and across the gate and source of the field-effect transistor in the first set 9 closest to the output node of the first driving circuit 14.
In operation, the first driving circuit 14 applies a voltage to the gates of the field-effect transistors in the first set 9 so as to dispose the field-effect transistors in the first set 9 in a non-conducting state. The second detecting circuit 19 then detects a voltage level across the gates and sources of the field-effect transistors in the first set 9, and compares the detected voltage level with the predetermined threshold voltage level. The second delay circuit 17 introduces a delay into the detected voltage level. The delay introduced by the second delay circuit 17 ensures that the field-effect transistors in the first set 9 are all in the non-conductive state. Thereafter, the second logic circuit 13 enables the second driving circuit 15 to apply a voltage to the gates of the field-effect transistors in the second set 10 so as to dispose the field-effect transistors in the second set 10 in a conducting state. After the predetermined period, the second driving circuit 15 applies a voltage to the gates of the field-effect transistors in the second set 10 so as to dispose the field-effect transistors in the second set 10 in a non-conducting state. The first detecting circuit 18 then detects a voltage level across the gates and sources of the field-effect transistors in the second set 10, and compares the detected voltage level with the predetermined threshold voltage level. The first delay circuit 16 introduces a delay into the detected voltage level. Then, the first driving circuit 14 is enabled by the first logic circuit 12 to apply a voltage to the gates of the field-effect transistors in the first set 9 so as to dispose the field-effect transistors in the first set 9 in a conducting state. After the predetermined period, the whole operation is repeated.
Although the aforesaid conventional transistor-based switching device 1′ achieves its intended purpose, it requires the first and second delay circuit 16, 17 and the first and second detecting circuits 18, 19. This results in a larger size for the conventional transistor-based switching device 1′ and in higher fabrication costs. Furthermore, the predetermined delay time periods introduced by the first and second delay circuits 16, 17, when not accurate, the field-effect transistors in the first and second sets 9, 10 may conduct simultaneously, which results in a poor operating efficiency for the conventional transistor-based switching device 1′.